This invention relates to a method for control of excessive corrosion in phosphoric acid circuits by oxidation of reduced ion species in wet process phosphoric acid with elevation of the valence state of certain dissolved reduced ion species such as ferrous iron (Fe.sup.+2 .fwdarw.Fe.sup.+3), uranium (U.sup.+4 .fwdarw.U.sup.+6), and vanadium (V.sup.+3 .fwdarw.V.sup.+4), among others.
In this respect, phosphoric acid is commercially produced by one of two methods; "furnace" or "wet process". In the "wet process" method, the phosphate ore is contacted with a mineral acid, such as sulfuric, to extract phosphate values. In the process many other metallic compounds present in the ore are also dissolved by the acid and remain in the phosphoric acid solution. For a reduction in acid corrosivity it is necessary to raise the valence states of these and other metals. The raising of these ion valence states is called oxidation.
Phosphate ore used in the production of phosphoric acid typically goes through a variety of processing stages to upgrade the ore, by removing some impurities before introduction into a digester circuit for the production of wet process phosphoric acid. The character of the ore used from one mining site to another can change dramatically; that is, impurities levels can vary widely, without affecting the processing of the ore with sulfuric acid to produce phosphoric acid.
The oxidation state of several impurities which are soluble in the digestion system are thought to be affected by the impurity mineral source as well as the reductive capabilities of carbonaceous material which is contained in some ores. Although calcination can be a processing step for ore preparation, it is thought that the presence of carbon can have the effect of increasing the proportions of some reduced metal species in the resulting calcined ore and hence in the resulting acid in a production system. One purpose of calcination is to oxidize organic carbon in the ore to give an ore feed to the phosphoric acid digesters that yields a more filterable gypsum. However, in practice this removal of carbon is never complete as proven by analysis. Steps can be taken to improve the calcination by improving the reaction kinetics. These include increasing the calcination temperature and insuring excess oxygen in the gas phase. However, the demand for maintaining production rates, the limitation of averting fusion of the ore in the calciners and operating limits of the equipment pose practical limits. The presence of organic carbon in the original ore favored the stability of reduced forms of accessory minerals; the presence of residual carbon in the calcined ore reflects a reducing environment as has been confirmed by the fact that a very highly reduced wet process phosphoric acid is at times produced from the calcined ore. The metal species which are affected most in the phosphoric acid are iron, vanadium and uranium. With large amounts of carbonaceous materials in the ore the tendency is to produce the reduced ion form of these metals which is Fe.sup.+2, V.sup.+3 and U.sup.+4, respectively.
The addition of most chemical oxidants to a wet process phosphoric acid system is either relatively ineffectual or not desirable, and a prescribed kind of oxidation and its control are necessary. For example, aeration in a primary digester of a phosphoric acid circuit with a computed sufficient amount of air at near ambient pressure appeared to be fairly ineffectual in destroying the strong reducing property of the acid and in avoiding activation toward corrosion (creating passivation) of the stainless steel and other alloy components in the system. Nitric acid additions lead to the evolution of toxic fumes of nitrogen oxides and may lead to corrosion by another mechanism. Sodium chlorate, while a good oxidant, can cause corrosion by another mechanism due to the rise in chloride level in the acid, particularly if the addition is not well controlled. Even MnO.sub.2 additions, one of the embodiments of this invention, require proper control because the resulting Mn.sup.+2 reaction product in the acid can cause some deleterious effects related to insoluble components in fertilizer products made from the acid if its additions are not properly controlled.
One method of the present invention therefore provides oxygen autoclave oxidation of wet process phosphoric acid at specified process parameters including pressure, temperature, mixing conditions and location of treatment with maintenance of EMF through the digestion process to thereby significantly increase oxidation of reduced ion species in the system for corrosion control.
The effect of oxidative molecules or ions on corrosion rates on stainless steels and other alloys (passivation) has long been recognized. "Corrosion Resistance of Metals and Alloys," 2nd Ed. Reinhold Publishing Corp., New York, pages 397-400, 1963 (A.C.S. Monograph Series), discussed the corrosiveness of phosphoric acid, although the discussion refers to data on laboratory grade phosphoric acid, not wet process acid. To our knowledge, however, a successful method of the use of oxidants to limit corrosion in wet process phosphoric digester circuits had not been successfully evolved. This is attributed to the special problems of this impure acid slurry system, to the constant replenishment of reductants and reduced ions in the process from the ore, to the high levels of reduced species in solution where they activate corrosion and to the necessity of monitoring and controlling the oxidation process. The present invention achieves this corrosion control without incurring severe deleterious effects from oxidants and complements other established methods of corrosion control in phosphoric acid circuits including insuring an adequate supply of reactive silica in the digesters and limiting the level of chloride.
The invention describes a workable means of maintaining and controlling the system to limit corrosion by changing the composition of the wet process acid to a less corrosive solution. The invention also provides means of predicting the corrosiveness of the acid.